1. Field of the Invention
The present invention relates generally to signal processing. More specifically, systems and methods for sound pressure level limiter with anti-startle feature for systems such as those that play sound via an audio device located close to the listener's ear or via a loudspeaker or other transducer located distant from the listener are disclosed.
2. Description of Related Art
Proper control of acoustic signal levels in communications and other audio output devices is desirable to ensure high quality audio output and hearing comfort to the users. For example, a telephone headset provides a speaker contained within an earpiece positioned over the user's ear. To ensure acoustic safety and high acoustic quality, the sound level of the acoustic signal emitted by the speaker should fall within a specified sound intensity range. Above the specified intensity range, the excessive sound level may cause discomfort for the user and/or the user's hearing. Thus, excessively high sound levels are of particular concern in communication and other audio devices such as telephone handsets and headsets and other listening devices that position a speaker near the user's ear.
Excessively high sound levels may be caused by various events. For example, accidental disturbances within a communication connection, such as an amplifier malfunction, intense feedback, incorrect signal source, and/or a phone line shorted to a power line, may cause dramatic increases in the electrical signal level input to a transducer that converts electrical signals to acoustic signals. The transient time for the acoustic signal to reach excessively high levels may be very short, such that a user often does not have sufficient time to move the listening device away from the ear in time to prevent exposure to the high sound levels. Although a handset user may be able to quickly move the handset speaker away from the ear as the user is typically already holding the handset in the hand, it may take a hands-free headset user longer to bring the hand to the headset in order to move the headset earpiece away from the ear. Furthermore, headsets are particularly suitable for users who are on the telephone for long periods of time, e.g., telemarketers, receptionists, and operators. Thus, because of the extra time required to remove a headset away from the ear and the potentially longer periods of headset usage, headset users may be particularly vulnerable to exposure to excessively high sound levels caused by sudden or constant loud audible signals.
Many countries have legislation limiting the maximum sound pressure level (SPL) that telephone equipment, including headsets, may produce. Noise exposure legislation is intended to prevent noise-induced hearing loss. The legal maximum SPL is generally relatively high, e.g., approximately 118 dB SPL or 118 dB(A) SPL, and is extremely loud when compared with normal telephone speech. Thus, telephone handsets and headsets that comply with the law can nonetheless cause user discomfort due to loud sound levels and may also startle the telephone or headset user due to sudden increases in the sound level from relatively quiet to relatively loud.
Reducing or removing sounds that are significantly louder than normal speech, even those sounds below the legal limits, may help enhance the comfort of telephone or headset users. User comfort may also be improved by preventing acoustic startle, i.e., the involuntary contraction of bodily muscles resulting from unexpected moderate or intense acoustic stimuli with rapid onset. In a quiet environment, even sound levels as low as 50 dB SPL, similar to or below normal telephone speech levels, can cause acoustic startle.
Headsets and other audio output devices often employ audio limiting devices on the receiver input terminals in order to limit the voltage and thus the maximum sound level from the headset receiver. Most conventional audio limiting devices either clip or compress the electrical signal that drives the headset, which prevents the electrical signal from exceeding a specified peak-to-peak or root mean square (rms) voltage. However the sound pressure level produced by the headset is determined at least in part by the receiving sensitivity of the headset, which in turn depends on the headset model and can generally vary significantly with frequency. Thus current methods for clipping or compressing the electrical signal require that these worst case tolerances are accounted for, which may sometimes result in lower overall levels than are necessary or desirable.
Thus, it would be desirable to provide an audio output device that limits sounds that exceed a specified sound pressure level threshold and thus prevent discomfort caused by loud sounds. It would also be desirable to provide an audio output device that reduces the likelihood and intensity of acoustic startle.